The present invention relates to electroexplosive devices (EEDs) such as detonators, blasting caps and squibs. More particularly, this invention relates to a method and device for desensitizing EEDs to electromagnetic radiation and electrostatic charges, thus preventing the premature or inadvertent detonation thereof.
A variety of propulsion systems and ordnance depend upon an electrical signal to initiate combustion. This signal is typically a dc current. The current flows through a conductor (typically a bridgewire supported between two posts) which causes a rapid temperature rise via ohmic heating. Once the conductor reaches a sufficiently high temperature it ignites nearby material. The ignited material is then used to initiate combustion of secondary material. The device which consists of the conductor and primary combustionable material is referred to as an electroexplosive device (typically referred to as an EED or squib).
Over the past four decades the electromagnetic environment of an electroexplosive device has changed dramatically. The operation of high-power radar and communication equipment has introduced high-intensity electromagnetic fields to the environment.
The fields can be coupled into an electroexplosive device. The methods of coupling are direct radio frequency (RF) radiation (e.g., the EED acts as the load of a receiving antenna) and arcing associated with weapons procedures such as the attachment of an umbilical cable. These two events will be referred to as electromagnetic interference (EMI).
A prime hazard of the conventional EED is that a coupled signal (caused by either direct RF radiation or arcing) will heat the bridgewire sufficiently to cause accidental firing.
Additional difficulties associated with the bridgewire include an EED that will not fire after exposure to electromagnetic interference (EMI) or severe mechanical stress. The mechanical stress of the EED includes severe vibration during flight and transport, and thermal stress induced by heating and cooling as the EED changes environments.
The former failure results from the bridgewire burning in two at a "hot-spot". The latter is caused by the wire breaking off at a support post.
This disclosure discusses a novel EED structure which is inherently immune to stray electromagnetic fields whether they be directly coupled or caused by inadvertent arcing. Additionally, the structure will not fail to ignite after EMI exposure or severe mechanical stress.
Various methods have been used to alleviate the problem of misfiring caused by electromagnetic radiation. Prior art systems have included inductive and capacitive components that form a balanced bridge or a tank to shunt unwanted signals from the bridgewire. One such protection device is disclosed in Parker et al., U.S. Pat. No. 3,181,464 issued May 4, 1965, which employs special conductors. Parker is used with EEDs having an exploding bridgewire. Other prior art devices add discrete components such as capacitors and inductors to form RF filters or otherwise electronically shunt unwanted signals away from the bridgewire. For example, Jones, U.S. Pat. No. 4,304,184 uses one or more inductors and ferrite beads to oppose and/or absorb unwanted current flow. Proctor et al., U.S. Pat. No. 4,378,738 passes the leads through ferrite chokes.
These prior art devices are often unsuitable for commercial production because of high manufacturing costs, and the constant downsizing of ordinance requires a greater degree of miniaturization than is possible with ferrite material and/or discrete inductors or capacitors. The degree of protection required is also expanding as the radio frequency interference/electromagnetic interference (RFI/EMI) environment becomes more hostile, e.g., a carrier deck with various fire control and navigation radar systems sweeping the ordnance at close range. In this hostile environment all frequencies may have high power. The protection must be broad band and capable of handling high induced currents. It is also increasingly important that the conducting area between the protective device and the bridgewire be reduced as currents adequate to misfire can be induced in conductors or ever-decreasing size as power in the EMI/RFI environment increases.
Another corollary to the EMI/RFI environment becoming increasingly hostile to ordnance on the carrier decks is a need to protect all ordnance aboard the ship and associated aircraft and not just missiles and large explosives. Even the 20 and 30 millimeter cannon rounds, and for that matter, all electrically fired ammunition, independent of size or type, needs protection against environment induced firing. State of the art devices and techniques involving discrete components and/or ferrites are all too bulky to be incorporated in small calibre ordnance.
As a result, a new design for a miniaturized, highly effective protective device is needed.